1. Field of the Invention
This invention relates to direct conversion receivers, and more particularly to a method and apparatus for calibrating DC offsets in a direct conversion receiver.
2. Description of Related Art
As our society becomes more mobile, wireless communication devices become increasingly small. Reducing component size within portable wireless communication devices such as cellular phones, pagers and wireless personal digital assistants (PDAs) is increasingly desirable. Reducing radio receiver size provides a significant reduction in wireless communication device size. One exemplary radio receiver is a “direct conversion” receiver. Direct conversion receivers are also known as “zero intermediate frequency” (zero IF) receivers. Direct conversion receivers are well known in the wireless communication art. Direct conversion receivers are described in many prior art publications such as U.S. Pat. No. 4,944,025, issued on Jul. 24, 1990 to Gehring et al., U.S. Pat. No. 4,736,390, issued on Apr. 5, 1988 to Ward et al., U.S. Pat. No. 4,955,039, issued on Sep. 4, 1990 to Rother et al., U.S. Pat. No. 5,241,702, issued on Aug. 31, 1993 to Dent and U.S. Pat. No. 5,323,425, issued on Jun. 21, 1994 to Colamonico et al., which are all hereby incorporated by reference herein for their teachings on direct conversion receivers.
Direct conversion receivers are smaller in size than are typical radio receivers because direct conversion receivers require less tuned circuitry than do the other receiver designs. Typical radio receivers convert incoming signals to intermediate frequencies (IF) in an interim conversion prior to demodulation. In contrast, direct conversion receivers convert incoming signals directly into baseband signals in a single operation, thus eliminating the interim conversion step required by other types of receivers. Advantageously, direct conversion to baseband reduces the amount of tuned circuitry (e.g., it allows for the elimination of IF circuitry) typically required in radio receivers. A reduction in tuned circuitry allows a larger portion of the radio receiver to be integrated, and thus reduces the overall size of the radio receiver.
FIG. 1 shows a block diagram of a typical direct conversion receiver. As shown in FIG. 1, an antenna 10 is coupled to a low-noise amplifier (LNA) 12. The antenna 10 receives incoming radio frequency signals and outputs the signals to the LNA 12. An incoming signal typically comprises a modulation signal and a carrier signal. The LNA 12 amplifies the signal from the antenna 10. The output of the LNA 12 is coupled to the inputs of the In-phase (I) down converter 14 and the Quadrature-phase (Q) down converter 16. The converters 14, 16 split the signal from the antenna 10 in a well-known manner that generates an “in-phase” or I-channel signal, and a “quadrature” or Q-channel signal and down converts the input radio frequency to baseband. The incoming signals are thereby separated or split into the I-channel and Q-channel or more simply “I” and “Q” signals. These split signals follow two separate signal processing paths as shown in FIG. 1. Separate signal processing paths are utilized to determine the polarity of the modulation signal. Determining signal polarity in a direct conversion receiver by using two substantially similar signal processing paths is well known in the art and thus is not described in more detail herein.
As shown in FIG. 1, the local oscillator used by direct conversion receivers is typically implemented by a voltage-controlled oscillator (VCO) 19. The VCO 19 outputs a local oscillation signal to the input of a 90-degree phase shifter 18. The phase shifter 18 splits the local oscillation signal into In-phase (I) and Quadrature-phase (Q) local oscillator signals in a well-known manner. The phase shifter 18 outputs the I and Q local oscillator signals to the inputs of the I and Q down converters 14, 16, respectively. The I and Q down converters 14, 16 mix the I and Q local oscillator signals to generate baseband “in-phase” and “quadrature” output signals.
The I and Q down converters 14, 16 output down-converted baseband signals to respective I and Q baseband channel filters (i.e., In-Phase baseband filter 20, and Quadrature baseband filter 22). The I and Q baseband channel filters 20, 22, respectively, filter the I and Q baseband signals and output filtered I and Q baseband signals to respective automatic gain control (AGC) amplifiers 24, 26.
The AGC amplifiers 24, 26 preserve the linearity of the filtered I and Q baseband signals, and thereby aid in accurately recovering the modulation signal. The AGC amplifiers 24, 26 output the processed baseband signals to respective I and Q analog-to-digital converters (ADC) 28, 30. The ADCs 28, 30 convert the analog processed baseband signals to digital baseband signals. The I and Q digital baseband signals are output from the ADCs 28, 30 and provided as inputs to a digital signal processor (DSP) 32. The DSP 32 analyzes the digital I and Q baseband signals and recovers the modulation signal therefrom in a well-known manner. Due to their design, as described above, direct conversion receivers advantageously reduce the amount of tuned circuitry required to implement radio receivers. However, direct conversion receivers disadvantageously produce unwanted side effects.
Disadvantageously, as is well known, receivers are particularly prone to signal interference caused by DC offsets (i.e., standing voltages). Direct conversion receivers are prone to this type of interference because the local oscillation signal (e.g., the signal generated by the VCO 19 and provided as input to the I and Q mixers 14, 16 through the 90-degree phase shifter 18) is typically operating at approximately the same frequency as the incoming signal (i.e., the signal received by the antenna 10). Therefore, self-mixing of the local oscillator (i.e., the VCO 19) and the In-Phase and Quadrature mixers (i.e., the mixers 14, 16, respectively) produces DC offsets. Unless these DC offsets are removed from any DC-coupled baseband signal, they will appear as interference in the received signal.
The self-mixing term is highly dependent on the amount of leakage/radiation generated by the local oscillator and injected into the front-end of the receiver. Referring to FIG. 1, DC offsets are produced from several sources within the direct conversion receiver. For example, DC offsets can be caused due to imperfections in the I and Q down converters 14, 16 and due to leakage from the VCO 19 back to the antenna 10. In fact, the largest source of DC offsets is due to leakage from the VCO 19 back to the antenna 10. DC offsets often cause the AGC amplifiers 24, 26 to saturate, and thus produce degradation or total loss of the incoming signals. The self-mixing term can be much larger than the received signal. Therefore, unless removed, the DC offset will completely dominate the incoming signal. This is especially true if the incoming signal is relatively weak in comparison. One obvious approach is to design the receiver so that the local oscillator leakage and radiation levels are reduced to minimum discernable signals. However, in practice, this proves to be difficult to achieve. Alternatively, direct conversion receivers can correct for DC offsets using DC offset calibration techniques.
Some prior art methods for calibrating DC offsets in direct conversion receivers comprise measuring the DC offsets and adjusting incoming signals accordingly. Two difficulties are encountered when attempting to measure and calibrate DC offsets in direct conversion receivers. First, any disturbances introduced by the process of measuring the DC offsets will change the DC offsets that are measured thereby, and will consequently not be removed by the calibration process. Second, during the measurement phase, the desired incoming signal should not be present. Otherwise, energy generated by the desired incoming signal will be added to the measured DC offset, and will subsequently be erroneously removed thereby introducing errors into the received signal.
Therefore, any DC offset calibration method has two major goals associated therewith: (1) the reduction or minimization of disturbances that can change the DC offset during the measurement phase; and (2) the measurement of DC offsets without the presence of the desired incoming signal during the time of measurement. Such methods should take advantage of the observation that receivers in some wireless communication systems such as Time-Division Duplexing (TDD) and Time-Division Multiple Access (TDMA) systems are only periodically active (i.e., actively receiving signals). Time intervals in such receivers can be classified as either “active” or “inactive”. During active time intervals the receiver receives incoming signals. During inactive time intervals the receiver does not receive incoming signals and is therefore free to perform functions that are unrelated to receiving or processing the received signals, such as maintenance, system checks, and DC offset calibration.
One method for calibrating DC offsets in a direct conversion receiver measures DC offsets during inactive time intervals and adjusts the incoming signals accordingly. Referring to FIG. 1, the method begins calibration during inactive time intervals by first isolating the antenna 10 from the rest of the receiver. Isolation can be accomplished using switches that operate by switching the antenna 10 off. After switching the antenna 10 off, the method measures the DC offset in the receiver. Before the inactive time interval expires the antenna 10 is switched on to allow the receiver to once again actively receive incoming RF signals. Ideally, the DC offset is accurately measured and the method subtracts the DC offset from the incoming signals during active time intervals. However, disadvantageously, the method does not always produce ideal results.
For example, due to the isolation of the antenna, the method tends to disturb the DC offset measurement during the DC measurement interval and consequently inaccurately measures the DC offsets. In addition, due to inherent system timing uncertainties (and exacerbated by the goal of performing calibration measurements as close in time as possible to the beginning of the TDD or TDMA active time intervals), the method disadvantageously may measure DC offsets when a desired signal (e.g., a carrier wave or a modulated wave) is present. If the measured DC offset includes a desired signal (i.e., a carrier or modulated wave) the subtraction phase (i.e., the phase when the measured DC offset is subtracted from the received incoming signals during active time intervals) will disadvantageously remove a large portion of the desired signal together with the DC offset.
Therefore, a need exists for a method and apparatus for calibrating DC offsets in a direct conversion receiver. The method and apparatus should accurately measure DC offsets by reducing disturbances that change DC offsets during the measurement intervals. Such a method and apparatus should accurately measure DC offsets even in the presence of incoming signals. The method and apparatus should isolate the receiver from incoming signals during the time that DC offsets are measured. This isolation should be accomplished without switching the antenna off during the DC offset measurement phase. The present invention provides such a DC offset calibration method and apparatus.